1. Field of the Invention
The present invention relates to the field of gas gauges suitable for use in vacuum environments of a lithographic apparatus.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.
During lithography, a wafer, which is disposed on a wafer stage (WS), is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
The reticle is usually mounted on a reticle stage (RS) and generally located between the wafer and a light source. In photolithography, the reticle is used as a photo mask for printing a circuit on the wafer, for example. Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors (also known as computer chips), the key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistors can be formed on the wafer. A wafer with many transistors results in a more powerful, faster microprocessor.
As chip manufacturers have been able to use shorter wavelengths of light, they have encountered a problem of the shorter wavelength light becoming absorbed by the glass lenses that are intended to focus the light. Due to the absorption of the shorter wavelength light, the light fails to reach the silicon wafer. As a result, no circuit pattern is created on the silicon wafer. In an attempt to overcome this problem, chip manufacturers developed a lithography process known as Extreme Ultraviolet Lithography (EUVL). In this process, a glass lenses can be replaced by a mirror.
Photolithographic exposure tools map wafer topography in order to set focus. They typically employ an array of sensors lined up next to each other. Topographic data is taken form each one and stored, then an algorithm is employed to establish the best plane for the exposure step. However, optical means of determining focus positioning are subject to errors from interfering wave fronts from lower layers.
One alternate to the optical means is an air gauge since an air gauge does not suffer from the effects generally associated with optical means of determining focus positioning. An air gauge as an auxiliary focus sensor may be capable of detecting the topography of wafers with higher fidelity than the (optical) level sensor. Significant potential exists to expand the capabilities of the air gauge to the point that it becomes a viable replacement for a leveling sensor. Besides being a more accurate metrology device, particularly in an optically noisy environment of processed wafers, it is considerably less expensive and it occupies a significantly smaller volume.
Among developmental challenges to be overcome before such advancement can take place, there appears to be two prominent challenges. Firstly, since a typical air gauge has a relatively long response time, it limits the useful bandwidth to approximately ˜50 Hz. Secondly, the fluidic response of the air gauge moving over the wafer topography with finite velocity may need adequate optimization of various controlling parameters. For example, shortening the response time of the air gauge requires a faster mass flow sensor (an internal component of the air gauge) and possibly shrinking the volume of the air passages.
Moreover, next generation lithography machines may use a vacuum environment to eliminate absorption losses and contamination. Operating an air gauge in these conditions will change the pneumatic operating conditions from low speed viscous flow to high speed, reaching sonic conditions. The high speed will produce much larger gas flow than can be accommodated in a vacuum environment and simple inlet throttling will reduce the bridge flow rates to levels that cannot be measured.